Sedimentology

موضوع: Sedimentology 8th ديسمبر 2010, 1:01 am

Sedimentology encompasses the study of modern sediments such as sand,[1] mud (silt),[2] and clay,[3] and the processes that result in their deposition.[4] Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary structures.[5]

Sedimentary rocks cover most of the Earth's surface, record much of the Earth's history, and harbor the fossil record. Sedimentology is closely linked to stratigraphy, the study of the physical and temporal relationships between
rock layers or strata.

The premise that the processes affecting the earth today are the same as in the past is the basis for determining how sedimentary features in the rock record were formed. By comparing similar features today to features in the rock record—for example, by comparing modern sand dunes to dunes preserved in ancient aeolian sandstones—geologists reconstruct past environments.
There are four primary types of sedimentary rocks: clastics, carbonates, evaporites, and chemical.

* Clastic rocks are composed of particles derived from the weathering and erosion of precursor rocks and consist primarily of fragmental material. Clastic rocks are classified according to their predominant grain size and their composition. In the past, the term "Clastic Sedimentary Rocks" were used to describe silica-rich clastic sedimentary rocks, however there have been cases of clastic carbonate rocks. The more appropriate term is siliciclastic sedimentary rocks.
o Organic sedimentary rocks are important deposits formed from the accumulation of biological detritus, and form coal and oil shale deposits, and are typically found within basins of clastic sedimentary rocks
* Carbonates are composed of various carbonate minerals (most often calcium carbonate (CaCO3)) precipitated by a variety of organic and inorganic processes. Typically, the majority of carbonate rocks are composed of reef material[citation needed].
* Evaporites are formed through the evaporation of water at the Earth's surface and most commonly include halite or gypsum.[6]
* Chemical sedimentary rocks, including some carbonates, are deposited by precipitation of minerals from aqueous solution. These include jaspilite and chert.